A device that is capable of both transmitting and receiving optical signals in a fiber-optic network is called a fiber-optic transceiver. There are two operational modes of communication in a bi-directional transceiver: the transmission (Tx) mode, and the reception (Rx) mode. In the Tx mode, a transmitter typically converts an electrical input signal to an optical signal by modulating a laser or light-emitting diode (LED) source. The optical signal is coupled to an optical fiber and transmitted to the optical fiber network. In the Rx mode, a receiver receives an optical signal from the optical network, and converts it into an electrical signal through the use of a photodetector. In a bi-directional transceiver, only one optical fiber is used for transmitting and receiving optical signals. The fiber is multiplexed in such a way that it can both accept the incoming optical signal from the distant optical source, as well as to carry the outgoing optical signal from the local source to the network.
Signal attenuation is commonly expressed in terms of its dimensionless decibel loss:dB=−10 log A/B where A is the attenuated signal and B is the original signal. Thus, for example, if an attenuated signal is 50% of a transmitted signal, the foregoing equation would be:dB=−10 log 0.5/1.0=−10 log 0.5=−3 dB.In other words, the loss of half of the original signal represents a −3 dB loss in signal strength.
Conventional methods of accomplishing bi-directional communications in an optical network typically use either a fiber-optic coupler or an optical beamsplitter. However, these methods have one common drawback, namely, that at least 50% of the optical power is typically lost in transmitting and receiving the optical signals. The losses occur because both the coupler and the beamsplitter are only partially transmissive or reflective. One can adjust the transmission ratio for the two fiber-optic branches in the coupler, or the transmittance and reflectance in the beamsplitter, but a theoretical loss of about −6 dB will occur.
To further understand the theoretical losses, assume that a light source has an optical power of 1 mw, and assume that each fiber-optic coupler or beamsplitter has a transmittance and reflectance of 0.5, and has no excess losses. In the transmission mode, only about 0.5 mw will be transmitted through the first coupler or beamsplitter to the outgoing fiber. As demonstrated above, this represents a total loss of about −3 dB. In the distant transceiver, the received signal will be further halved as it again passes through the second coupler or beamsplitter. This represents another loss of about −3 dB, and a total loss of −6 dB in passing through two couplers or beamsplitters. Thus, approximately 75% of the optical power is lost in communicating between two conventional transceivers. An additional −0.3 to −0.5 dB optical power is lost if excess losses are considered.
Accordingly, it would be generally desirable to provide an improved fiber-optic transceivers that may be used in an optical fiber network, and that have improved transmission and reception efficiencies.